In the third aliyah Bilam saddles his donkey and goes with the Moavite dignitaries. However, G-d places an angel with a drawn sword in Bilam's way. Rashi says that animals have better sight than humans, so the donkey could see the angel while Bilam could not. The Torah describes the trouble Bilam has trying to get the donkey to go forward. G-d finally gives Bilam the ability to see the angel. Bilam sees him and becomes afraid and offers to go back. But the angel says he can go on, but only to say what the G-d wants him to say.